1. Field of the Invention
The invention relates to liquid crystal compositions, temperature indicating devices incorporating such compositions as a functional component thereof and methods of their manufacture.
2. Brief Description of the Prior Art
Liquid crystal compositions and their use in a wide variety of temperature sensing and recording devices are well known in the art; see for example the disclosures of U.S. Pat. Nos. 3,578,482; 3,594,126; 3,697,297; 3,704,625; 3,720,623; 3,941,901; and 3,974,317. Liquid crystals are compounds which lack a distinct melting point between solid and isotropic liquid states. Instead they exhibit an intermediate mesomorphic phase between the solid and the liquid state. In for example, cholesteric liquid crystals the mesomorphic phase (sometimes referred to as the "mesophase") is characterized by a highly colored form due to light scattering by the periodic structure (plane texture) of the phase. The color phenomena associated with the mesophase of the cholesteric liquid crystal is a function of its structured chirality or "twist" and is advantageously exploited in a variety of sensing and recording devices such as temperature indicators, electro-optical displays and the like; see for example U.S. Pat. Nos. 3,720,623; 3,779,751; and 4,016,094. When warmed to a temperature above its liquid transition point, the cholesteric liquid crystals pass from the mesomorphic phase through a phase transition point to an isotropic liquid form which may have a different color (or be colorless) than exhibited in the mesophase state.
The term "liquid transition point" as used throughout the specification and claims means the specific event at which a liquid crystal compound of liquid crystal composition passes from the mesomorphic phase to the form of an isotropic liquid as measured in terms of the temperature. This temperature event is also referred to occasionally by those skilled in the art as a "clearing point".
Thus, if one knows the liquid transition point for a given cholesteric liquid crystal, one can know when the associated temperature has been reached by the color change which occurs at the liquid transition point.
Subsequently, when the liquid crystals in their isotropic liquid form are cooled, they may not immediately revert to the colored state associated with the mesomorphic phase. Instead, they may exhibit a "hysteretic memory" which allows, for example, a temperature excursion to be observed for some time after it has occurred.
The prior art temperature sensing devices generally function by observation of the color displayed when the liquid crystal composition passes from the mesophase through its liquid transition point. For many compositions, the time period required for passage back to the mesophase upon cooling of the liquid is brief, hence the time period for observation of the liquid transition point or liquid state as an indicator of some physical occurrence such as a temperature range achievement is also brief. It would be advantageous for certain temperature indicating devices if the color or lack of color associated with the liquid state (light-scattering condition) of a given indicator composition could be retained for extended periods of time, to facilitate observation that the liquid transition point has been reached and exceeded. For example, if this extended period, which is termed the "hysteretic recovery time" could be lengthened or delayed, the operator-observer would have greater latitude in making observations that a certain sensed temperature range has occurred. Thus, there would be an improvement in for example, clinical thermometers employing a liquid crystal composition as a temperature sensor.
The present invention is based on the discovery that there is a relationship between the hysteretic recovery time phenomena exhibited by liquid crystals and the viscosity of the liquid crystal composition in the liquid isotropic state. This was not previously appreciated in the prior art where the prior art temperature sensing compositions only rarely approached viscosities in their liquid state as high as about 10 poise, and certainly not higher.
The compositions of the invention are particularly advantageous as temperature sensing components in the construction of liquid crystal functioning devices such as temperature sensing and recording instruments, where it is desirable to modify or prolong the hysteretic recovery character of the liquid crystal component, while retaining quick initial response times (response to a temperature event). Other advantages and uses of the invention will be discussed more fully hereinafter.